Ornamental design for a duck



FIG. 1 is a top view of a duck showing my new design;

FIG. 2 is a left side view of the design;

FIG. 3 is a front view of the design;

FIG. 4 is a rear view of the design;

FIG. 5 is a right side view of the design; and

FIG. 6 is a bottom view of the design. 

The ornamental design for a duck, as shown and described. 